The present invention relates to sintered cemented carbide bodies coated with thin and extremely wear resistant surface layers. The invention also relates to a method of making such coated bodies.
It is known that the wear resistance of pressed and sintered cemented carbide bodies, as for example, inserts for chipforming machining, can be increased considerably by applying hard surface layers. In particular, coatings of metal carbides, metal nitrides or metal oxides have been applied as thin layers (having, for example, a thickness between 1 to 20 .mu.m) on the cemented carbide core or the substrate. It is also known that further advantages can be reached in certain cases by using a thin coating composed of two or several different layers applied on top of each other, for example, the use of a carbide or a nitride as an intermediate layer below an outer ceramic layer. Aluminum oxide (Al.sub.2 O.sub.3) and zirconium oxide (ZrO.sub.2) are examples of such ceramic surface layers. One main method of applying the surface coatings is the CVD-technique "Chemical Vapor Deposition", in which the coating is deposited on a hot substrate by reaction between gaseous components. For the production of aluminum oxide coatings, the most common chemical vapor deposition system which has been employed utilizes the hydrogen reduction of aluminum chloride, which is either evaporated directly, or formed by the reaction between aluminum metal and chlorine or hydrogen chloride, and the reaction with water vapor, which is either evaporated directly or formed by the reaction between hydrogen and carbon dioxide, or oxygen.
Suitable hard, polycrystalline, compact and well-adherent coatings of aluminum oxide possessing the desired wear-resistant properties are normally only obtained at deposition temperatures above about 950.degree. C. At lower deposition temperatures, loose, powdery deposits are usually obtained which deposits consist of the gamma and/or theta modifications of aluminum oxide. At deposition temperatures of about 1000.degree. C. and above, the aluminum oxide phase which has normally been identified and found suitable for cutting tools is the alpha form of modification. This form of aluminum oxide is, however, a high temperature phase which is normally not expected to be produced in a pure state by chemical vapor deposition at a deposition temperature below 1000.degree. C. The stability of alpha aluminum oxide deposited at temperatures below 1000.degree. C. is dependent upon the presence of impurities or dopants either coming from the substrate which is being coated or from the gas phase. When using pure alpha aluminum oxide substrates, epitaxial growth of alpha aluminum oxide by chemical vapor deposition only takes place at deposition temperatures above about 1500.degree. C.
From the above, it is clear that there is a considerable risk of obtaining multiphase aluminum oxide coatings at the temperatures normally used in the production of coated tools. In a multiphase coating, the boundary regions between the various phases will constitute regions of considerable mechanical weakness and they can therefore be the cause of premature tool failures.
The deposition of an aluminum oxide coating involves the diffusion of various species from the substrate and/or the gas phase. The interplay of the various diffusion, nucleation and growth mechanisms which govern the formation of the coating is very delicate and can easily result in the formation of inhomogeneous deposits. Since such mechanisms are often difficult to control, a process which can provide a stable, specific aluminum oxide phase would be most advantageous as cemented carbide bodies having a single-phase aluminum oxide coating are expected to have a superior and more consistent performance when compared to multiphase aluminum oxide coatings.